The present invention relates to anodes for use in controlling corrosion and more particularly to providing an improved protective shield and sheathing for such anodes which will not interfere with the normal corrosion protection afforded by the anode but which will prevent accidental impact between the anode and another metallic object.
Typically the corrosion of metals in sea water and other aqueous environments is an electrochemical process, wherein a flow of electrons takes place between certain areas of the metal surface in contact with the aqueous solution which is capable of conducting an electric current. The result of this electrochemical process is deterioration or eating away of the metal in those areas which are commonly referred to as anodes and at which the electrons leave the metal. Metal ions enter into the solution and the metal corrodes. The area receiving the electrons and becoming more negative is termed the cathode. There electrons are discharged to the sea water electrolyte. The cathode also must be present to complete a circuit. These two electrodes may comprise different metals or different areas on the same piece of metal due to impurities on the surface, differences in surface structure, etc. A common and well-known example of such an electrochemical action is the dry cell battery principle which provides energy to make the flashlight operate. Another similar example is an unprotected hull of the ship in sea water, the primary difference being that there is no switch to cut off the flow of electrons and, furthermore, the presence of the reaction is undesirable. In the case of a ship's hull, since the amount of electrons the earth can accept is near infinity, the process will continue until the hull is completely wasted away and, as long as the flow of electrons is unimpeded through a path of low resistance, the rate of wastage will be very rapid.
This electron flow is generally termed a galvanic process. It will tend to continue in the absence of the application of an equal or greater opposing force, which if applied, can greatly reduce the rate of the galvanic process or corrosion can be stopped completely. This is accomplished by supplying a sufficient number of electrons from another more powerful source so that the supply will make the structure to be protected the cathode. The application of such an opposing force would provide the surplus of electrons without a loss of steel and is well known as cathodic protection. Using this type of protection means being prepared to sacrifice another material or energy for that purpose, which is the primary function of cathodic protection. By permitting the electrons from the galvanic anode to flow to a tanker hull or tank surface which has a more positive potential, i.e. cathodic, the desired protective function is obtained. The rate of electron flow depends on the driving force or potential difference between the metals, i.e. the voltage difference between the corroding site known as the anode and the protected site known as the cathode.
In the case of unprotected ballast tanks in tankers, corrosion also takes place although there is no dissimilarity in metals. The corrosion is substantial because of the basic princple involved which is the same as explained above in the ship's hull example. Thus, surface imperfections, orientations in granular structure of the metal, lack of homogeneity, localized stresses and variations in geometry or shape and environment cause the formation of large numbers of localized anodes and cathodes on the surface of the metal. The galvanic action results in anodic corrosion, i.e. corrosion at the anode site in comparison to the protected site which is the cathode. This corrosion is commonly prevented, or at least minimized, by using sacrificial anodes which are typically cast on support rods or cores and fixed at various locations throughout the tank in order to provide for complete protection. Hence, it is often desirable to place anodes at many elevations from top to bottom. Generally, the schemes employed for mounting the anodes are regulated by the U.S. Coast Guard. Presently various ship classification societies and the U.S. Coast Guard permit installation of aluminum anodes only in the lower levels of cargo/ballast tanks and other locations where a free fall of the anode cannot result in an impact of more than 200 foot pounds energy. Impacts of greater energy between aluminum and rusted steel can produce hot aluminum fragments capable of igniting petroleum gases existing above the liquid level or when the tank is free of liquids. Present practice permits installation of the zinc anodes at greater heights because impacts between zinc and steel are less hazardous.
It is apparent that in order to achieve a completely safe anode environment system, it is desirable to completely eliminate any metal-to-metal impact possibilities. The prevention of impact should encompass not only those resulting from accidental detachment of the anode but also impact from an object falling onto the anode from above. If these objectives can be met then the use of aluminum anodes will be safe at any elevation in the tank.
Various attempts at protection of anodes have been proffered in the prior art. These include those such as disclosed in U.S. Pat. No. 3,488,275 which provide protection from physical abuse during shipping in the form of a fabric container for the anode. U.S. Pat. No. 2,976,226, generally discloses providing a sleeve on an anode to protect against oil or reaction products but not impact; however, the anode is of the impressed current type. U.S. Pat. No. 3,196,101 provides a mesh wrapped about an impressed current anode for protecting it against any falling pieces, but does not suggest protection against metal-to-metal impact.